Is Your Weight Loss Making You Fat?

Yep, you read that right. Allow us to explain.

You’ve probably heard that if you lose a ton of weight, your metabolism is going to slow down. Now a new study shows that even modest weight loss affects the thyroid hormone activity in your body, making it harder to continue to lose weight—and could even set you up for regaining what you’ve lost and then some.

The study, by researchers at the National Institutes of Health, found that overweight people who lost 5 to 10% of their body weight (an average of about 15 pounds) over the course of a year had a corresponding decrease in serum T3 levels. T3 is the body’s “active” form of your thyroid hormone and it is what fires up your metabolism.

The catch: Most blood tests for thyroid function won't pick up on a dip in T3. That's because they look for T4, the form of thyroid hormone secreted by the thyroid gland, which needs to be converted into T3 to become active. (Study subjects didn't experience a significant change in T4 levels.)

So what does this mean for you? It could help explain why, after an initially successful weight loss, you hit the dreaded plateau, says Francesco Celi, MD, lead study author and an NIH clinical investigator.

But before you think a thyroid medication is the answer, not so fast. “Because T3 levels were still within a low-normal range, we would not consider this a reason for giving people any type of replacement thyroid hormone,” he says. “It’s not clear that it would be helpful in this sort of case, and I am concerned that it could be harmful. Overtreatment with thyroid hormones can suppress normal thyroid function and cause cardiac problems and bone loss.”